1. Field of the Invention
A method utilized in a wireless communications system and related communication device are disclosed, and more particularly, to a method of enhancing positioning measurement in a wireless communications system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
Toward advanced high-speed wireless communications system, such as transmitting data in a higher peak data rate, LTE-Advanced system is standardized by the 3rd Generation Partnership Project (3GPP) as an enhancement of LTE system. LTE-Advanced system targets faster switching between power states, improves performance at the cell edge, and includes subjects, such as bandwidth extension, coordinated multipoint transmission/reception (COMP), uplink multiple input multiple output (MIMO), etc.
COMP is considered for LTE-Advanced as a tool to improve coverage of high data rates, cell edge throughput, and system efficiency, which implies dynamic coordination among multiple geographical separated points. That is, when an UE is in a cell-edge region, the UE is able to receive signal from multiple cells, and the multiple cells can receive transmission of the UE.
For certain reason, such as emergency consideration, several positioning methods are developed for UE positioning estimation. Positioning methods supported within the E-UTRAN include cell ID based method, observed time difference of arrival (OTDOA) method, network-assisted Global Navigation Satellite System (GNSS) method, etc. Please note that, the OTDOA positioning method may be assisted by network configurable idle periods (namely OTDOA-IPDL), in order to potentially improve the hearability of neighbouring eNBs. The OTDOA positioning method involves measurements made by a UE, and the measurement results are then sent to the network (e.g. E-UTRAN) for UE positioning estimation. Generally, the measurements are based on the arrival time difference from different cells observed/measured at the UE.
In the LTE-Advanced system, the network includes features of coordinated multipoint transmission/reception (COMP), and provides assistance data to the UE for the positioning measurement. The assistance data includes physical cell identities (PCIs) of candidate cells, and relative transmission timing of candidate cells to a serving cell of the UE or to each candidate cell itself. The PCI is used for identifying each of the candidate cells. Moreover, since distances between the UE and the candidate cells are different, the propagation delays between the UE and the candidate cells are varied as well. Thus, the relative transmission timing is required for the UE for estimation of reference signal (RS) arrival time. Please note that, the reference signal is transmitted within a positioning subframe that the network configures to a candidate cell, wherein the positioning subframes are based on normal subframe with no data or Multimedia Broadcast multicast service Single Frequency Network (MBSFN) subframe with no data.
Therefore, according to the PCIs provided in the assistance data, the UE knows which cells are participate in the positioning measurement, and measures reference signals from ones of the candidate cells, thereby generating a measurement report to the network. Thus, the network can perform the UE positioning estimation based on the measurement report.
However, due to a numerous deployment of relays and home base stations, the number of available PCIs used for cell identification is insufficient for the number of cells in the LTE-Advanced system. In other words, there may be more than one cell sharing the same PCI, thereby causing PCI confusion problem. For example, the UE measures a references signal of a cell, which is provided in the assistance data and has the same PCI as other cells, and sends the measurement report to the network. In this situation, the network is confused by the PCI derived from the measurement report, and cannot understand which cell the PCI is referred. That is, the network does not know which of the cells with the same PCI was actually measured by the UE. Thus, the network may use wrong related transmission timing and wrong location of the cell with the same PCI for estimation of the UE positioning.
Moreover, due to unclear specification for the positioning measurement in the LTE-Advance system, several scenarios are described as follows.
In the first scenario, when the UE measures a RS from a cell whose PCI is not provided in the assistance data, the LTE-Advanced system does not clearly specify how the UE shall deal with this situation. The measured cell may not be configured by the network for the positioning measurement. Since the cell is not provided in the assistance data, the related transmission timing corresponding to this cell is not given in the assistance data either. Thus, the UE cannot know what related transmission timing should be used for estimation of the RS arrival time.
In the second scenario, RS interference may occur among the candidate cells. The UE may consider a detected signal as a RS from a candidate cell, which is actually the interference. However, the LTE-Advanced system does not clearly specify how the UE determines the detected signal is for the position measurement or just interference, thereby impacting UE position calculation. The UE may make a wrong interpretation of the reception of the RS, and mislead the network.
In the third scenario, according to the features of the LTE-Advanced system, the cells of the LTE-Advanced system shall be coordinated and synchronized for the positioning measurement. That is, the time alignment information of the cells may be known by the network. However, the UE may measure a certain cell which has no synchronization interactions with those synchronized cells. Take a home base station as an example. Due to operation of the home base station, a closed subscriber group (CSG) cell is associated with a small coverage base station which may be deployed for magnificent advantages, such as the high bandwidth wireless internet access in the home and office, and efficient and cost-effective capacity solution for indoor coverage. The small coverage base station may provide restricted access to only UEs belonging to a Closed Subscriber Group (CSG) and usually serve several CSG cells. The CSG cell has no interface to any of the normal cells in the LTE-Advanced system, and is not synchronized to these cells. Thus, the network has no time alignment information of the CSG cell. Without the time alignment information, the network may not have sufficient information for positioning measurement of the UE which may access the CSG cell, thereby causing UE positioning calculation inaccuracy.